This invention relates generally to means enabling faster light output from a metal halide discharge lamp and more particularly to a combination of anode and cathode means in a metal halide lamp promoting more rapid light output during lamp start-up.
Various metal halide discharge lamps commonly employ a fused quartz arc tube as the light source by reason of the refractory nature and optical transparency of this vitreous ceramic material. In such type lamps the arc tube generally comprises a sealed envelope formed with fused quartz tubing with discharge electrodes being hermetically sealed therein. A typical arc tube construction hermetically seals a pair of discharge electrodes at opposite ends of the sealed envelope although it is known to have both electrodes being sealed at the same end of the arc tube. The sealed arc tube further contains a fill of various metal substances which becomes vaporized during the discharge operation. The fill includes mercury and metal halides along with one or more inert gases such as krypton, argon and xenon. Operation of such metal vapor discharge lamps can be carried out with various already known lamp ballasting circuits employing either direct current or alternating current power sources.
For rapid sustained illumination with metal halide lamps, such as a xenon-metal halide lamp, a performance requirement now exists for at least fifty percent of the steady state light output to be reached within 0.75 seconds from the moment of lamp start-up. The prior art lamps experience significant light loss during start-up when the xenon discharge illumination is either absorbed or scattered by mercury which condenses upon the arc tube walls when first vaporized from the discharge electrodes. A "light hole" thereby results between the xenon illumination and less rapid illumination being produced by vaporization and ionization of the mercury and other metal ingredients further contained in the arc tube. By minimizing the light hole in these prior art lamps, a more sustained or continuous source of illumination is thereby provided. We have found considerably more mercury condensing on the conventional cathode electrodes than condenses on the conventional anode electrodes when these lamps are cooling down. The light hole occurs on the next lamp start when mercury is almost immediately vaporized from these cathode means which are also found to heat much faster than the conventional anode means. Recondensation of this vaporized mercury on still cooler arc tube walls during the lamp restart period produces a transitory light-blocking film which is located principally between the spaced-apart electrode means.
Accordingly, it is an object of the present invention to provide means whereby metal halide lamps experience less light loss during start-up.
Another object of the present invention is to provide an improved metal halide lamp employing a fused quartz arc tube as the light source which includes means for reduction of mercury condensation on the arc tube walls.
It is a still further object of the present invention to provide an improved automotive headlamp employing a metal halide lamp as the light source which experiences less light loss during start-up.
These and other objects of the present invention will become apparent upon considering the following more detailed description.